Noise reduction system for radar



June 3, 1952 Franz.

W. W. HANSEN ETAL NOISE REDUCTION SYSTEM FOR RADAR Filed April 19, 1946 SWEEP CVRCU/ T FUL L WAI VE REC TIF/ER PSS F/L TER MPL /F/ER CL /PPER Patented June 3, 1.952

ors-wiso sursis is The prese'nt interi-tin Arelates gn'iliy to 'Staus `y objects. meteoritos fs'igiiono Y'from Aniovfsf.' moors, howeveri have -fiequooisumffofing fore 'these signals 'pass -thro'uh the vlter. f

vO 'the moving I Th,

in frequency by the full-wave rectifier and added in the indicating circuit to the other applied signals in such phase and amplitude as to substantially cancel a periodic noise wave present therein as a result of the keying of the receiver.

The invention in another of its aspects relates to novel features of the instrumentalities described herein for achieving the principal objects of the invention and to novel principles employed in those instrumentalities, whether or not these features and principles are used for the said principal objects or in the said field.

Other objects and advantages wil1 become apparent from the specification taken in connection with the accompanying drawings wherein:

Fig. 1 is a block diagram of a distance measuring radio system embodying the present invention; and

Figs. 2-13 are graphs of idealized wave shapes illustrating operation of the structure of Fig. l.

In Fig. 1 the direction of control or energy flow is represented by arrows, while in Figs. 2-13 vertical and horizontal axes represent wave amplitude and time, respectively, drawn to an arbitrary scale.

The principle underlying the present invention is the realization that noise in an intermittently responsive radio receiver has a modulation envelope corresponding in shape to the responsetime characteristic of the receiver. This modulation results in the generation by a non-linear circuit, such as a rectifier, of a periodic noise Wave at the keying frequency of the receiver which wave has an amplitude not only dependent upon the magnitude of the desired signal but also sufciently great to seriously impair the sensitivity ,of the receiver.

' The essence of the invention is the provision of apparatus for producing in the indicating circuit of the intermittently responsive receiver a leads I2 and I3, respectively. Lead I2 is connected to an ultra high frequency transmitter I4 which is adapted to be turned on for alternate half cycles of the square wave. The lead I3, on the other hand, is connected to a radio frequency amplifier I5, mixer I6- and intermediate frequency amplifier I'I forming a portion of a receiver which is made operative by the wave on lead I3 in phase opposition with the wave controlling the transmitter, i. e., the operations of the transmitter and receiver are alternate, the receiver being insensitive when the transmitter is radiating and the transmitter being quiescent when the receiver is sensitive.

The transmitter cycle is represented by portions I8 of the keying wave illustrated in Fig. 2, while the operating cycle of the receiver is shown by portions I9 of the keying Wave-illustrated in Fig. 3. In practice, the receiver is not switched on until a short time after the transmitter is switched off. Likewise, the transmitter is not switched on until after the receiver is switched off. This delaying action prevents overlap between the two functions. The repetition rate is so chosen that a delay occurs during the transit time of radio propagation to and from objects at the maximum desired range equal to approximately one-half the keying period.

The relatively long radio pulse waves generated by the transmitter I4 are fed through a coaxial line 2l to a directive radiator 22, represented as a parabola. Associated with the coaxial line 2| are a TR box 23 which prevents the high powered transmitted pulses from entering the receiver circuits and an RT box or anti TR box 24 which prevents any of the weak received signals from returning to the transmitter I4 and being lost therein. If the length of transmission line from the RT or ATR box 24 to the T joint adjacent the antenna is a multiple of a half wavelength, then when the transmitter is off, the RT switch 24 gap will be an open circuit, which is reflected to the T joint of the transmission line as an open circuit, thereby channelling received signals to the RF amplifier I5. The radio frequency energy is radiated in the form of la beam which when it strikes an object results in a reflection, some reflected energy returning to the radiator 22. The received signals pass through the TR box 23 with little attenuation and are applied to the radio frequency amplifier I5. After amplification, the radio signals are supplied to the mixer I6 to which is also supplied a local oscillator frequency from a local oscillator multiplier 25 over a lead 26. The local frequency on lead 26 always differs by a constant intermediate frequency from the transmitted frequency on lead 21 because both are different harmonics of a basic oscillator frequency which is generated and multiplied in local oscillator multiplier 25.

The output wave of the mixer contains signals corresponding to the received radio signals, merely vhaving been translated to a convenient intermediate frequency band. The intermediate frequency amplifier I1 continues the amplification process and supplies the amplified wavesto a second detector 3| into which is also introduced a reference intermediate frequency wave from the local oscillator multiplier 25 over a lead 28.

The second detector 3l reproduces in the presence of a stationary reflecting object pulse waves shown, for example, at 32 in Fig. 5. These waves are caused by the combined action of the transmitter and receiver keying waves. The duration of the wave 32 depends upon the range of the Y object. The front edge 33 of the wave 32 represents the moment when the receiver is switched on while the rear edge 34 represents the end of the transmitted pulse wave. The time required for the transmitted pulse wave to travel to the refiecting object and return results in a delay in the returned wave relative to the transmitted wave as illustrated, for example, by pulse wave 35 in Fig. 4. This delay causes a portion of the returned wave to arrive after the receiver has been made sensitive. Therefore, the width of the resultant pulse wave 32. shown in Fig. 5, or the phase of the fundamental component of this pulse wave relative to the keying wave is a measure of the distance to the object. Although the width of the pulse Waves due to stationary objects is a function of the distance, it is to be noted that the periodicity of the pulse waves remains unchanged, merely the number and amplitude of the harmonies being altered. These pulse waves, therefore, contain only the fundamental and higher harmonics of the keying frequency.

A reflection from a moving object, for example, at the same range as the stationary object represented by the wave form 35 of Fig. 5 is shown at 36 in Fig. 6. Here it is to be observed that the wave 36 consists of a Doppler beat note due to the relative motion of the object which note is modulated or keyed at the keying frequency and higher harmonics. The lowest and largest components 1n the ywave $6 shown 6 lare the fDoplQler frequency LAani 'fthe T-Dep'p'ler frequency 'pl'us vfthe unamental of V'the `izeying frequency and "the Dopierv'frequency -nilnus `the fundamental* or the k'eyin'grequency. Y

The waves detected in the se'conll lietelfstorjtl are impressed vupon la fba'nd-'stop Vfilter @il 'which is ldesigned "to suppress 'the 'fl'lncailiental `0f the keying `Afrequency 'and its harmonics. "Ihiis, no sienai'siueltosubstantially'stationary objeotslare transmitted-ty the filter. lon 'the other nana, Signals due -to Objectshauiii'g substantial *rlaial velocities eare'ireely passed by'lter 31 'sinc'eitliese signals do not contain the tkeying"freq'ue'nc'y.

Signals that "pass tll'rouglfl` lte'r v3-1 are V'applied toa fu'lllwaverectler. The aetion'ofrectifler 3s fon the weve es of 'Flg. is `is Y"'illustrated fas ln'lig. '1. `vrThe negative Ahalf of 'thewave \3`f`is inverted and ythe resultant Wave inlay be "seen to comprise f the fundamental keyin'g requency, v'the fundamental tkeying frequency plus and 'minus twice `the Doppler 'frequency and many other higherharmonics and combinations. AVaryfnariowband-pas's filter-'eil fis conneetedltofth'e out put 'circuit of rectifier'be to `eliminatesubst'anf tiall-'y Lallfrequency components exceptvthefunialnenta'l vvkeying :frequency which is `shown by wavein in-'Fig.'8. Thephase of'thislfundaietal component wave `is dependent upon J'the instance to thelinoving lilfet.

An amplifying and clipping-circuit #33 is feonneotecl to "the lilarrow band-'pass lter 1"4I lt`o cenvert the fundamental wave 42 issiiirigfrom ithe filter f4! into -al1square Wave 44 Shown inFig. 9. iA "pulse 'Wave fgeneiator "45 attached to Vthe a'm'- plier Clipper 43 differentiates tlii'ssduare 'wave 13.4 to produce sll'arppulse waves 46 as indicated inflFig. n10. These pulse waves 4S arie `applied to "a -Leathode #ray indieator 41 for the purpose v.of intensifying Vthe Aelectron `beam threin. '-The -bearnfisswet across theindieato'iffalceby alinear sweep circuit "4'8 whichis controlled /b'y Ltl'i'e `wave `Illsup'plied overa leaddll `f-rintliekever Il. The position c-f a'nli'ntensiled Aimage on `the indicator falce is 'atieord-ingly representative 5o`f the tin'e delay between the transmitted .pulse 'wave and received-pulse wa'veonwhatis equivalentto the distance v-to the :reflecting moving object.

The indi'catingeircuitiisresponsive ite the Iam'- plitlide anii Tpliese "of thelsl'ibstantially sinusoidal wave `which is itransmitted `-by the -narrow bandpass llter 4 l. Thei'iidi'c'ator f4-1 is thlis 4-eajpable ef gi-vi'ng a true distanceindieatienff only one objeotata time. When two objects `are isilnuk taneoisly within '-the Irradiation pattern 'of the 'systr'm ltt'vo sine Waves having fanplitudes -V'cle- {Jenden-tuponlmany factorsiriclildingthe amount of reflected energy returned from tll'erespeetive objects-'and'phases dependent only uponth'e distance to the objects are combined vin theindicating-circuit 4to yield an image on 'the screen of 'the Aindicator 41 'whose-position fluetuatesaccording to the varying'resultantof Ythe two variable-compo'liet waves. This eiieet is Azrliniiniaed llypro- -vidin'g a highly directive Jladiant beam.

The ultimate sensitivity of the lradio ll'receiifer is determined, as 1in 'all electrical isystens, -by fthe ratio iof the lstrength yoi `the v`desired Signal to `the fstrength o'f the interfering noise impressed `on the indicator 41. Thep'resent invetrsha've beenable'to demonstrate by-ma'thema'tical analyss 'and confirm by experimental investigation :thattne noise disturbances present in thei'ndi- 'eating fcircuit consist fof, 'r'st, a 'continu'ous jlirequency A"spectinll lofi random n'ise petentials and,

fili

seconl, al'periouie lnoise etvefirevi'ifg:*iraniency equ to lthe ffuneanientai 'oi the keying *wave The 'irenom noise has 'an average interference power propotionafl 'ft'o the vvrelit'leney band of the narrow band-pass fllteif'dl. die noise Wave arises f'i-n 'rectler 338 'as a ultof the square wave f envelope lmpre'sse'd y the vvinf-A terniitten't opera'tonfoiffthe -receiverfon the 'n tfans'mittetl throug'n f'bane'pa'ss ter 131.l y phase o'f'thef'li dic noise Wa'veiis-sii tle'saineas i-thephase f'o'f aflt'fave 'fc'ali'sei'byv`r potlietical xjno'v'in-g oojeo't at la irai-lee correspo ingitoiatransittime cleiay-offfone-n'if the ne ou o'fftlie keyingwave.

This periodic noise 'wa-ve -adds -vectrlfalllylvvith thefdes'ireti signal-ann tenes to profili fai "nge error 'iin fthe measurement of the ti unlesscancelled 'according' toftheftea" vpresentflnve'ntion. 'The power-ration 'e oiiicn'oise 'relative to' tli'e l'raricloi'nfnoi'se'stzli proportionaltothekeyingiirequency offk 'er aniinvers'ely proportionartoth'e 'frequenc wdthof the narrow `inland-pass viilterdl. revident"that when,fas`is=usu1,'thereceiverkeymg 'frequency is "apprecially fgreater th'ettn -ith'e tii width 'of -the ffilter 4f! the fperiodi'c *noise fc' be utes inuoh :more l'iiils'eiferen'ce 3in 1tile inl ating' circuit than does the-randomnois'e'aniitheiornler disturbanceii-sithe :primarfy'ifaotorfin eet mining 'the weakest signaiftowiiic'hithe raiiio systern'ean satisfactorilyf'iespon.

A .f urther chatact'eristi'c 'ftiiis perioiicenliise disturbance is that because-"theietiiieri'blie altes V`as a linear =deteetor, tlle 'fnoiee rtenis to 35e fsuppres'seti V'when fa signal fis ipresent. "Fire linee# 'nituie fof "-tniszn'oise V:wave ohan'ges -with l'the strength Iof tnesignaiffiomf a moving obj'eet. "The `fangeerror fof fa'nioving robject, fthere'fore, Avvaries according "to Ith'e uc'tliatio'n 'in amplituief the reiiection from the object. 'This "vari n 'in error appears Y as an irregular l'inovenlent fof 'fthe image oil'the indicator screen.

The 7constant fre eney Santip'hase V'ofithe rp'erie 'odic noise wave init's lits cancellation by an artificial signal o e same frequency, opposite `planeand equaiielnpiiftude. is neceesarylrour ever, to introduce tlie artificial `isignaloefore the rectifier KV33 5in order -'that the fanioituiie -f noise eancelling "vol-'tage wary 1in iinison with -'the amplitude 'ofth'e eriodic :noise -wave :proce ities, ftlle'irequired noise cancelling signal is lloro- -halli'fthe:kev-u gto-every ""ele. e time A o fsunoient'ly great fso f-that the iias lon the n-'condueting leIectron tube is i'still -too:'negettive :a terfone 'nurse 'wave 453 has rbeen :generated r'to discharge Vei'fter vthe -fnext Ieontrol fpulse -wa-'ve f8 lias been :applied over lead 30. only "when fthe :succeeding 'pulse wave frs is impressed :fon -`.the circuit has the negative bias foil ftne :non-eww i'ueti've tuloe turned to 'aliotentiel low :encuen another output pulse Wave 53, The rectangular wave 53 thus generated has a fundamental component shown at 54 in Fig. 12 Whose peak coincides with the middle of alternate transmitter keying pulse waves I8.

An amplitude control 55 permits adjustment of the output wave from divider such as to yield maximum noise cancellation. The noise cancelling signal is applied from control circuit 55 over lead 56 to filter 3l. Since the noise cancelling wave is one-half the repetition frequency, it freely passes through filter 37, is converted to the keying frequency by full-Wave rectifier 38, and accompanies the desired signal through the narrow band-pass filter 4l.

The action of the full wave rectifier 3B is to invert the negative half cycles of the Wave 54 as shown by dashed lines 5l in Fig. 12. The fundamental frequency of this full-Wave rectified sine wave is illustrated by Wave form 58 in Fig. 13. This Wave 58 is twice the frequency of the original wave 54 applied to the full-Wave rectifier 38 and is, therefore, equal to the keying frequency. The wave 58 is seen to be in phase ,oppositionV with the interfering periodic noise wave shown by dashed line 59 and is entirely suitable to balance it out leaving only random no ise in the indicating circuit.

A further advantage of introducing|- the noise cancelling wave before the filter 3l is that any change in the transmission characteristics of the filter will act on the noise cancellation signal in a manner similar to the action of this circuit on the periodic noise Wave, thus assuring that the cancellation wave undergoes the same amplitude and phase fluctuations as are introduced in the noise wave which is to be cancelled. The filter 31 further eliminates all but the fundamental and third harmonic of the noise cancelling wave.

In order to aid the understanding of the operation of the present invention, some of the design factors involved in a practical system are now discussed. In one specific radio' system embodying the invention, '750 megacycles Was chosen for the carrier frequency. At this carrier frequency, a velocity of one mile per hour toward the radio system produces a Doppler beat note of approximately two and one-quarter cycles per second. A repetition rate or keying frequency of 1,000 cycles per second permitted the receiver to respond to objects moving at speeds ranging from very low velocities to over 400 miles per hour Without overlapping the Doppler frequency band with the keying frequency. At this repetition rate, the distance corresponding to maximum sensitivity of the receiver, Where the delayed reflected Wave coincides with the receiver keying wave, is approximately forty-six miles. A satisfactory width for the radiant beam was found to be about eight degrees between half-power angles and 100 Watts of peak transmitting power gave the system ability to pick up moving aircraft at ranges up' to sixty miles over terrainwhich yielded strong reiiections from stationary objects.

The band-pass iilter 4i was designed to have very high attenuation, say 80 decibels, and in- 'eluded a low-pass filter which greatly attenuated all frequencies above those which the filter was designed to pass. The noise rejection band pass filter 4i was tuned to a thousand cycles per second and had a band Width of only about four cycles per second. Not only were the filters stable in their characteristics but the phase shift throughout the pass bands was arranged to be such that a moving object at a particular range was indicated at substantially the same position on the screen of indicator 41 irrespective of any variation in its radial velocity.

Since many changes could be made in the above construction and many apparently Widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

i. In combination, means for producing a series of pulse signals, keying means coupled to said first-named means for alternately activating and de-activating said first-named means at a predetermined frequency, a band-stop filter coupled to the output of said first-named means for suppressing signals having frequencies substantially equal to the fundamental of said predetermined frequency, a full wave rectifier coupled to the output of said band-stop filter, a bandpass filter coupled to the output of said full Wave rectifier for passing signals having a frequency substantially equal to the fundamental of said predetermined frequency, a generator coupled to the output of said keying means for producing a signal having one-half the frequencyv of said predetermined frequency, and means coupling the output of said generator to the input of said band-stop filter.

2. In a receiver for receiving pulse modulated radio frequency waves, the combination of phase detecting means for producing pulse signals in response to the radio frequency signals received thereby, a keying signal generator, means coupling the output of said keying signal generator to said detecting means for alternately activating and de-activating said detecting means in synchronism with the signal produced by said keying signal generator, a band-stop filter coupled to the output of said detecting means for suppressing signals having frequencies substantially equal to the fundamental of the signal produced by said keying signal generator, a full Wave rectifier coupled to the output of said band-stop filter, a band-pass filter coupled to the output of said full wave rectifier for attenuating all signals except those having a frequency substantially equal to the fundamental of the signal produced by the keying signal generator, a frequency divider coupled to the output of said keying signal generator for producing a signal having onehalf the frequency of and having substantially the same Wave form as the signal produced by the keying signal generator, and means for coupling the output of said frequency divider to the input of said band-stop filter.

3. The apparatus of claim 2 further including means'for controlling the magnitude of the signal produced by said frequency divider.

4. The apparatus of claim 2 further including indicating means coupled to the output of said band-pass filter.

5. In combination, transmitting means for radiating electromagnetic energy tov/ard reiiecting objects, receiving means including a phase detector for receiving and detecting electromagnetic energy reflected from said objects, a keying signal generator coupled to said transmitting means and to said receiving means for alternately activating the transmitting and receiving means, a band-stop filter coupled to the output of said receiving means for suppressing signals having frequencies substantially equal to the fundamental of the signal produced by said keying signal generator, a full wave rectifier coupled to the output of said band-stop lter, a bandpass lter coupled to the output of said full Wave rectifier for passing signals having a frequency substantially equal to the fundamental of the signal produced by said keyingr signal generator, generator means coupled to the output of said keying signal generator for producing a signal of one-half the frequency of the signal produced by said keying signal generator, and means coupling the output of said generator means to the input of said band-stop filter.

6. The apparatus of claim 5 further including indicating means coupled to the output of said band-pass lter and to the output of said keying signal generator for comparing the phase difference between the electromagnetic energy pulses received by said receiving means and the electromagnetic energy pulses radiated by said transmitting means.

WILLIAM W. HANSEN. EDWARD J. BARLOW.

10 REFERENCES CITED The following references are of record in the lile of this patent:

UNITED STATES PATENTS Number Name Date 1,487,451 Farrington Mar. 18, 1924 1,894,656 Barons Jan. 17, 1933 2,055,883 Terry Sept. 29, 1936 2,222,739 Bole Nov. 26, 1940 2,227,057 Blumlein Dec. 31, 1940 2,310,692 Hansel Feb. 9, 1943 2,402,464 Suter June 18, 1946 2,406,316 Blumlein et al. Aug. 27, 1946 2,407,000 Evans Sept. 3, 1946 2,408,742 Eaton Oct. 8, 1946 2,423,088 Earp July 1, 1947 2,424,263 Woodyard July 22, 1947 2,446,244 Richmond Aug. 3, 1948 2,479,568 Hansen Aug. 23, 1949 

